I put tandem motors on the MIT test stand and had the curves.
There was a very short and not that violent blip when the booster motor
burned through and then a slight dip as the booster propellant was
completely consumed and the upper stage was just coming up to thrust.
The perfect example of the burn through spike that Bob is trying to describe is the AVI Gold Series motors. Let's look at two examples that used a looong 18mm casing and a looong 24 mm casing. They were basically booster motors and then they had a seperate smaller casing containing a delay train and ejection charge that was epoxied into the top. When first produced, the motors were extremely long and the 18mm was a full 20 Ns and the 24 mm was a full 40 Ns (unless I'm remembering wrong and they were just short of "full"). Anyway, when they burned, the casings got really thin from erosion. Not as bad as if they had been tandems, but still extremely thin. Some burned through the sidewall, but MANY blew through the sidewall as the propellant burned out. The forward face of the propellant would ignite as soon as the flame front blew through and since the aft face was already burning, it had a momentary spike of at least double the thrust. Since it was not open on top, the pressure equalized almost instantly on both sides of the rupture, so you did not get giant chunks of propellant breaking off and sending the peak thrust/pressure explosively high. However, the peak was enough to blow out the casing at it's thinnest.
Solution: they de-rated the motors, cutting about 5 Ns off each one and making the casings shorter. They also painted inhibitor on most of the forward face of the propellant before epoxying in the delay/ejection housing.
Estes had to do some casing magic to get the E9 motor to work with the long burn time, but they did not have to worry about the burnout spike, since they have integral delays.
-Fred Shecter NAR 20117
There was a very short and not that violent blip when the booster motor
The perfect example of the burn through spike that Bob is trying to describe is the AVI Gold Series motors. Let's look at two examples that used a looong 18mm casing and a looong 24 mm casing. They were basically booster motors and then they had a seperate smaller casing containing a delay train and ejection charge that was epoxied into the top. When first produced, the motors were extremely long and the 18mm was a full 20 Ns and the 24 mm was a full 40 Ns (unless I'm remembering wrong and they were just short of "full"). Anyway, when they burned, the casings got really thin from erosion. Not as bad as if they had been tandems, but still extremely thin. Some burned through the sidewall, but MANY blew through the sidewall as the propellant burned out. The forward face of the propellant would ignite as soon as the flame front blew through and since the aft face was already burning, it had a momentary spike of at least double the thrust. Since it was not open on top, the pressure equalized almost instantly on both sides of the rupture, so you did not get giant chunks of propellant breaking off and sending the peak thrust/pressure explosively high. However, the peak was enough to blow out the casing at it's thinnest.
Solution: they de-rated the motors, cutting about 5 Ns off each one and making the casings shorter. They also painted inhibitor on most of the forward face of the propellant before epoxying in the delay/ejection housing.
Estes had to do some casing magic to get the E9 motor to work with the long burn time, but they did not have to worry about the burnout spike, since they have integral delays.
-Fred Shecter NAR 20117